The only way somebody truly changes behavior is if some drastic consequence occurs because of their behavior. Being upset with your boyfriend and telling him to get a watch isn't working right? Then you've got to try something different.
I'm a person who's very serious about punctuality and being on time. I could never date anybody who was constantly late or constantly making ME late. I'd dump that person. That's how much I value my time. Now, that's probably as drastic as drastic will get and I can't in good faith recommend that. For one, I don't condone breaking up with somebody to prove a point, especially when it has the chance to backfire.
My suggestion would be to set his arse up. Next time he's late, leave. Or better yet, stop inviting him places. If you have somewhere to be and he's going to make you late, go without him. Or even tell him that he's not invited because he can't show up on time. Not that you're going to invite some other guy somewhere but let him know how serious it is to you.
Beating a kid doesn't always work but if you take away something they love, they get the message. Which takes us back to the good lovin'. If he won't act right, put him on a lovin' timeout.
And after you do the hardwork, if he still doesn't learn, then you have to decide how important it is to you. Perhaps he just can't ever be on time and you need to figure out if that's something you can deal with. Me? Absolutely cannot. But then again, I have like 75 watches.
It was written.